The fire-retardant for wood, HL-N, is made by compounding high-purity guanidinium uronic phosphate (GUP), boric acid, and a small amount of additives in a certain proportion. Utilizing the patent technology from Sichuan University's State Key Laboratory of Polymer Materials, GUP with high purity is synthesized at a high yield through the complete reaction of dicyandiamide and phosphoric acid in an aqueous solution, along with a solvent recycling process. No pollutants are generated during the production process.